GASTROINTESTINAL STROMAL TUMOR (GIST) OF THE RECTUM

S.A. Rooholamini, M.D., F. DeGregorio, M.D.*, M. Itani, M.S. VI, S. Saki, M.D., A. Matin, M.D., A.H. Au, M.D., G.C. Hansen, M.D., M.H. Lee, M.D., J. Rahimian, Ph.D., R.S. Saul, M.D., R.C. Verma, M.D.

From the Departments of Radiological Sciences and Pathology* at Olive View-UCLA Medical Center, Sylmar, CA

CASE SUMMARY

This is a 39-year-old man who presented with 9-month history of a painful mass in the posterior perineum originating from the anal region and extending into the base of the penis.  The mass is moderately painful and has been gradually increasing in size.  He has mild dysuria with no other urinary, gastrointestinal, or constitutional symptoms.  Specifically there is no nausea, vomiting, diarrhea, change in bowel habits, melena or hematuria.  The patient’s past medical history is unremarkable except for a motor vehicle accident with trauma to the left leg resulting in below knee amputation.  His family history is negative.  Patient is a social drinker, nonsmoker, and has no history of drug abuse.

On physical examination, a firm mass is palpable in the posterior aspect of the perineum on the right side.  The mass extends from the anus to the base of the scrotum, and measures approximately 6 cm.  On digital examination there are no mucosal lesions, but the mass is hard and tender on palpation.  Urinalysis is negative, and PSA level is 0.6.

RADIOLOGIC FINDINGS

An outside computed tomography (CT) of the abdomen and pelvis shows diffuse thickening of the wall of the rectum due to a mass measuring 6x5 cm in size, suggestive of a rectal neoplasm.  Transrectal ultrasound shows enlargement of the prostate with an abnormal contour and a hypoechoic cavity inside.  The wall of the cavity is thick and irregular.  A biopsy of the prostate was done.  A staging CT of the chest, abdomen, and pelvis shows a large soft tissue mass in the pelvic floor, measuring 8x7 cm, with a necrotic center.  The mass is contiguous with the anterior wall of the rectum and the base of the prostate, and extends into the perineum (figures 1-3).  There were small mediastinal, paraaortic and pelvic lymph nodes.

Figure 1.  Contrast enhanced axial CT image of the pelvis at the level of the base of the bladder shows a large mass in the region of the seminal vesicles and anterior aspect of the rectum.
Figure 2.  Contrast enhanced axial CT image of the pelvis at the level of the symphysis pubis shows a soft tissue mass with an area of low attenuation, probably due to necrosis.
Figure 3.  Contrast enhanced axial CT image of the pelvic floor demonstrates an abnormal soft tissue mass involving the anus and extending anteriorly into the base of the penis.

PATHOLOGIC FINDINGS

A biopsy of the lesion revealed no prostatic tissue, but cellular spindle cell neoplasm, and the immunohistochemical staining was consistent with GIST (figures 4 - 7), with positive CD117, vimentin, PIN-4, and weakly positive desmin (Table I).  The differential diagnoses of spindle cell tumors based on immunohistochemistry are tabulated in Table II.

The patient was maintained on Gleevec with a good response up to 21% reduction in the largest diameter of tumor (7.5 to 5.9 cm).  Patient was also referred for general surgery, however, he refused an abdomino-perineal resection, and he is being followed up.

TABLE I.  Immunostains Results in the Present Case

TABLE II.  Differential Diagnosis of Spindle Cell Tumors Based on Immunohistochemistry

*Caption Abbreviations:        GIST: Gastrointestinal Stromal Tumor
      SFT: Solitary Fibrous Tumor
      STUMP: Stromal Tumors of Uncertain Malignant Potential

PATHOLOGIC DIAGNOSIS

Gastrointestinal stromal tumor of the rectum.

 Click each image to enlarge.

Figure 4.  GIST on needle biopsy with palasaded pattern simulating Antoni A pattern of neural tumor (hematoxylin-eosin stain, original magnifications X200).
Figure 5.  GIST on needle biopsy with neuroid pattern (hematoxylin-eosin stain, original magnifications X200).
Figure 6.  GIST with spindle cell morphology simulating smooth muscle tumor (hematoxylin-eosin stain, original magnifications X200).
Figure 7.  Diffuse C-Kit (CD117) staining on needle prostate biopsy of GIST (hematoxylin-eosin stain, original magnifications X200).

DISCUSSION

Gastrointestinal stromal tumors (GIST) are rare tumors of the gastrointestinal (GI) mesenchyma, constituting less than 3% of all GI malignant neoplasias, and less than 1% of malignancies involving the rectum.  We present a case of rectal GIST along with its clinical and radiologic findings.

Gastrointestinal stromal tumors (GIST) are submucosal tumors of the gastrointestinal tract which derive from the interstitial cells of Cajal.  The Cajal cells are interstitial cells which have a pacemaker role to regulate the peristaltic activity of the GI tract.  These tumors have a mesenchymal origin and are not synonymous with leiomyomas or leiomyosarcomas of the gastrointestinal tract.  GIST are often large, bulky, lobulated, and exophytic.  Clinically they present as a mass.  Nausea, vomiting, and gastrointestinal bleeding are among the symptoms.  Bleeding occurs as a result of central necrosis.  These tumors have an excellent prognosis if they are small and completely excised.  If they are larger than 5 cm they have a poor prognosis.  These tumors respond well to chemotherapy when metastases are present.  The pathological diagnosis requires electron microscopy.  Approximately 66% of these tumors arise from the stomach.  The second most common place is the duodenum, but they can arise from the small bowel, colon or rectum.  Leiomyomas are more common in the esophagus than GIST.  Primary GIST of the prostate is very rare and only two cases have been reported in the literature.

Upper gastrointestinal series show a large intramural mass of the stomach which is exophytic and may show ulceration.  On computed tomography, calcifications of the mass may be visible in 25% of the patients.  The tumors are hypervascular or hypovascular on contrast enhanced CT and necrosis may be present.  Tumors are large, lobulated and exophytic.  The sensitivity of detection of GIST on contrast enhanced computed tomography is mentioned to be 93% and the specificity 100%.  Differential diagnoses would include lymphoma, sarcoma, other exophytic tumors of the gastrointestinal tract and submucosal lipomas.

CONCLUSION

A rare case of GIST of the rectum is presented and the clinical, the imaging characteristics, and the pathological diagnosis of these tumors are discussed.

REFERENCES

1.   Emedicine.  Gastrointestinal Stromal Tumors - Leiomyoma/Leiomyosarcoma.  Author: Vu Nguyen, MD.  Last updated: December 1, 2004.  http://www.emedicine.com/radio/topic388.htm

2.   Lee C-H, Lin Y-H, Lin H-Y, Lee C-M, Chu J-S.  Gastrointestinal stromal tumor of the prostate: a case report and literature review.  Human Path 2006; 37:1361-1365.

ACKNOWLEDGMENT

The authors would like to express their thanks for the invaluable assistance and devotion of René Retana for his computer and secretarial work, Paul Beers, Shervin Ghamghami, and Peter Ghazi for their technical work, and Marsha Kmec and Armineh Harutunian for their literature search in preparation of this work.